Hydrodynamic brush seal

ABSTRACT

A hybrid brush seal is provided having two bundles of axially spaced seal bristles each of which are mounted in a ring shape on a first machine component with bristle ends directed at a sealing surface of the second, rotating machine component. The bristle ends are kept from direct contact with the rotating machine component via one or more shoes which are designed such that as the shaft rotates a hydrodynamic film separates the shoe(s) from the shaft. The shoe(s) is attached to the bristle ends at discreet locations. Alternatively, one or more bundles of seal bristles are mounted at one end either to the fixed or rotating machine component, with the opposite bristle ends directed toward one or more shoes, and wherein one or more spring elements are connected between the machine component and shoes.

This application is a continuation of application Ser. No. 10/832,053,filed Aug. 26, 2004, now abandoned, which claims priority under 35U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 60/466,979filed May 1, 2003 for all commonly disclosed subject matter. U.S.Provisional Application Ser. No. 60/466,979 is expressly incorporatedherein by reference in its entirety to form a part of the presentdisclosure.

FIELD OF THE INVENTION

This invention relates to seals for sealing a circumferential gapbetween two machine components that are relatively rotatable withrespect to each other, and, more particularly, to a hybrid brush sealhaving two sets of axially spaced seal bristles each of which aremounted in a ring shape on a first machine component with bristle endsdirected at sealing surface of the second, rotating machine component.The bristle ends are kept from direct contact with the rotating machinecomponent via one or more shoes which are designed such that as theshaft rotates a hydrodynamic film separates the shoes from the shaft.The shoe(s) is attached to the bristle ends at discreet locations.Alternatively, one or more sets of seal bristles are mounted at one endeither to the fixed or rotating machine component, with the oppositebristle ends directed toward one or more shoes, and wherein one or morespring elements are connected between the machine component and shoes.

BACKGROUND OF THE INVENTION

Turbomachinery, such as gas turbine engines employed in aircraft,currently is dependent on either labyrinth (see FIGS. 1A–1E), brush (seeFIGS. 2A and 2B) or carbon seals for critical applications. Labyrinthseals provide adequate sealing, however, they are extremely dependent onmaintaining radial tolerances at all points of engine operation. Theradial clearance must take into account factors such as thermalexpansion, shaft motion, tolerance stack-ups, rub tolerance, etc.Minimization of seal clearance is necessary to achieve maximum labyrinthseal effectiveness. In addition to increased leakage if clearances arenot maintained, such as during a high-G maneuver, there is the potentialfor increases in engine vibration. Straight-thru labyrinth seals (FIG.1A) are the most sensitive to clearance changes, with large clearancesresulting in a carryover effect. Stepped labyrinth seals (FIGS. 1B and1C) are very dependent on axial clearances, as well as radialclearances, which limits the number of teeth possible on each land.Pregrooved labyrinth seals (FIG. 1D) are dependent on both axial andradial clearances and must have an axial clearance less than twice theradial clearance to provide better leakage performance than steppedseals.

Other problems associated with labyrinth seals arise from heatgeneration due to knife edge to seal land rub, debris from hardcoatedknife edges or seal lands being carried through engine passages, andexcessive engine vibration. When seal teeth rub against seal lands, itis possible to generate large amounts of heat. This heat may result inreduced material strength and may even cause destruction of the seal ifheat conducted to the rotor causes further interference. It is possibleto reduce heat generation using abradable seal lands, however, they mustnot be used in situations where rub debris will be carried by leakageair directly into critical areas such as bearing compartments or carbonseal rubbing contacts. This also holds true for hardcoats applied toknife edges to increase rub capability. Other difficulties withhardcoated knife edges include low cycle fatigue life debits, rubinduced tooth-edge cracking, and the possibility of handling damage.Engine vibration is another factor to be considered when implementinglabyrinth seals. As mentioned previously, this vibration can be causedby improper maintenance of radial clearances. However, it can also beaffected by the spacing of labyrinth seal teeth, which can produceharmonics and result in high vibratory stresses.

In comparison to labyrinth seals, brush seals can offer very low leakagerates. For example, flow past a single stage brush seal is approximatelyequal to a four knife edge labyrinth seal at the same clearance. Brushseals are also not as dependent on radial clearances as labyrinth seals.Leakage equivalent to approximately a 2 to 3 mil gap is relativelyconstant over a large range of wire-rotor interferences. However, withcurrent technology, all brush seals will eventually wear to line on linecontact at the point of greatest initial interference. Great care mustbe taken to insure that the brush seal backing plate does not contactthe rotor under any circumstances. It is possible for severing of therotor to occur from this type of contact. In addition, undue wire wearmay result in flow increases up to 800% and factors such as changes inextreme interference, temperature and pressure loads, and rubbing speedsmust be taken into account when determining seal life.

The design for common brush seals, as seen in FIGS. 2A and 2B, isusually an assembly of densely packed flexible wires sandwiched betweentwo plates. The free ends of the wires protrude beyond the plates andcontact a land or runner, with a small radial interference to form theseal. The wires are angled so that the free ends point in the samedirection as the movement of the runner. Brush seals are sized tomaintain a tight diametral fit throughout their useful life and toaccommodate the greatest combination of axial movement of the brushrelative to the rotor.

Brush seals may be used in a wide variety of applications. Althoughbrush seal leakage generally decreases with exposure to repeatedpressure loading, incorporating brush seals where extreme pressureloading occurs may cause a “blow over” condition resulting in permanentdeformation of the seal wires. Brush seals have been used in sealingbearing compartments, however coke on the wires may result inaccelerated wear and their leakage rate is higher than that of carbonseals.

One additional limitation of brush seals is that they are essentiallyuni-directional in operation, i.e., due to the angulation of theindividual wires, such seals must be oriented in the direction ofrotation of the moving element. Rotation of the moving element or rotorin the opposite direction, against the angulation of the wires, canresult in permanent damage and/or failure of the seal. In the particularapplication of the seals required in the engine of a V-22 Ospreyaircraft, for example, it is noted that during the blade fold wing stowoperation, the engine rotates in reverse at very low rpm's. This isrequired to align rotor blades when stowing wings. This procedure isperformed for creating a smaller aircraft footprint onboard an aircraftcarrier. Reverse rotation of the engine would damage or create failureof brush seals such as those depicted in FIGS. 2A and 2B.

One attempt to limit wear of brush seals is disclosed in U.S. Pat. No.5,026,252 to Hoffelner in which a sliding ring is interposed between thebristle pack of the seal and the moving element or rotor to avoid directcontact therebetween. The bristle ends are received within acircumferential groove in the sliding ring and are allowed to freelyfloat or move within such groove. Although bristle wear may be reducedin this design, it is believed that the seal created at the interface ofthe sliding ring and rotor is unsatisfactory.

An improvement of prior brush seals, including that disclosed in the'252 patent to Hoffelner noted above, is found in my U.S. Pat. No.6,428,009. In that design, one end of each of a plurality of sealbristles is fixed in an annular shape and mounted to the fixed machinecomponent or stator while their opposite ends are attached to a numberof individual shoes located proximate the rotating machine component orrotor. Prior to shaft rotation, the shoes are in contact with the rotorsurface with preferably the leading edge of each shoe set to have lesscontact than the trailing edge of the shoe. When the rotor begins torotate, a hydrodynamic wedge is created which lifts the shoe slightlyoff the surface of the shaft allowing the shoe to effectively float overthe shaft at a design gap. It has been found that one limitation of thedesign disclosed in the '009 patent is a potential problem with “rollover” under pressure load, i.e. the shoes can tip or pivot in the axialdirection thus creating a leakage path.

Carbon seals are generally used to provide sealing of oil compartmentsand to protect oil systems from hot air and contamination. Their lowleakage rates in comparison to labyrinth or brush seals are well-suitedto this application, however they are very sensitive to pressurebalances and tolerance stack-ups. Pressure gradients at all operatingconditions and especially at low power and idle conditions must be takeninto account when considering the use of carbon seals. Carbon seals mustbe designed to have a sufficiently thick seal plate and the axial stackload path must pass through the plate as straight as possible to preventconing of the seal. Another consideration with carbon seals is thepotential for seepage, weepage or trapped oil. Provisions must be madeto eliminate these conditions which may result in oil fire, rotorvibration, and severe corrosion.

According to the Advanced Subsonic Technology Initiative as presented atthe NASA Lewis Research Center Seals Workshop, development of advancedsealing techniques to replace the current seal technologies describedabove will provide high returns on technology investments. These returnsinclude reducing direct operating costs by up to 5%, reducing enginefuel burn up to 10%, reducing engine oxides of emission by over 50%, andreducing noise by 7 dB. For example, spending only a fraction of thecosts needed to redesign and re-qualify complete compressor or turbinecomponents on advanced seal development can achieve comparableperformance improvements. In fact, engine studies have shown that byapplying advanced seals techniques to just a few locations can result inreduction of 2.5% in SFC.

SUMMARY OF THE INVENTION

A hybrid brush seal is provided which is generally similar to the onedisclosed in my prior U.S. Pat. No. 6,428,009, but which overcomes thetendency of the shoes to roll over under the application of a pressureload.

In one presently preferred embodiment, two sets or bundles of sealbristles are axially spaced from one another, i.e. in the direction ofthe longitudinal axis of two relatively rotating machine components suchas the rotor and stator of a gas turbine engine. One end of the sealbristles in each bundle is fixed in an annular shape to either thestator or the rotor, while the opposite end of the seal bristles in eachbundle extends to one or more shoes circumferentially disposed about theother machine component. The shoes are located with respect to the rotoror stator to created a seal between the two while avoiding contact ofthe seal bristles with the relatively rotating component. Each of theshoes is connected at discrete points to the end of the seal bristlessuch that the leading edge of the shoe is oriented to have less contactwith the rotor or the stator than the trailing edge of the shoe. In oneembodiment, each shoe is connected at two spaced locations to theabutting seal bristles by electron beam welding or similar mountingtechniques, thus creating two hinge points for the shoe to translateabout.

In an alternative embodiment, one or more bundles or seal bristles aremounted at one end to either the rotor or the stator, and their oppositeend extends toward one or more shoes located proximate the other of therotor or stator. A spring element is connected between the shoes and therotor or stator which is flexible in the radial direction, but axiallystiff. The spring element functions to assist in preventing roll over ofthe shoes with respect to the rotor or stator where it is located, thusmaintaining an effective seal under pressure load. It is contemplatedthat the ends of the seal bristles proximate the shoes can be eitherconnected to the shoes such as by welding or other means of attachment,or spaced from the shoes. In either case, the seal bristles act as asecondary seal between the rotor and stator in combination with theshoes.

In operation the shoes of either embodiment of this invention functionvery similarly to that of a tilting pad bearing shoe. Prior to rotationof the rotor, the shoe is in contact with the rotor or stator surface.Because the leading edge of the shoe has less contact with the rotor orstator than its trailing edge, when the rotor begins to rotate ahydrodynamic wedge is created that lifts the shoe slightly off of thesurface of the rotor or stator. Consequently, the shoe “floats” over therotor or stator at a design gap, such as 0.0005 to 0.0010 inches.

The advantages of the hybrid brush seal of this invention are many. Ithas the same sealing characteristics of existing brush seals, but willnever change in performance due to bristle wear. The brush seal backingplate can be moved further outboard of the I.D. because the shoeprevents the bristles from bending over in high pressure applications.Each shoe may have a certain amount of interference with the rotor orstator prior to rotation. Thus, the seal can be slightly off centerduring assembly but once rotation begins, each pad will lift-off. Hence,tight tolerances can be relaxed.

The hybrid seal of this invention can be utilized in all sealapplications, including labyrinth, brush and carbon. The robust designeliminates the careful handling now required of carbon seals utilized inlube system compartments. This seal may allow the engine designer toutilize less parts in the assembly as this seal will permit “blind”assemblies to occur.

The following table provides a comparison of the seal of the subjectinvention with currently available technology.

Dependence Contamination Seal Type Wear Rate Leakage on ClearancesPotential Labyrinth High Low High High Seals Brush Seals Medium LowMedium Medium Carbon Seals Medium Very Low High Low Hybrid Seal Low LowLow Low

DESCRIPTION OF THE DRAWINGS

The structure, operation and advantages of this invention will becomefurther apparent upon consideration of the following description, takenin conjunction with the accompanying drawings, wherein:

FIGS. 1A–1E are schematic views of a number of prior art labyrinthseals;

FIGS. 2A and 2B depict views of a prior art brush seal;

FIG. 3 is a cross sectional view of one embodiment of the hybrid brushseal of this invention;

FIG. 4 is a schematic, elevational view of the seal shown in FIG. 3;

FIG. 5 is a view similar to FIG. 4, except of an alternative embodimentherein;

FIG. 6 is a schematic, elevational view of an alternative embodiment ofthe seal herein employing a single bundle of seal bristles and axiallyspaced spring elements;

FIG. 7 is a view similar to FIG. 6, except employing two sets of axiallyspaced seal bristles;

FIG. 8 is a cross sectional view of a still further embodiment of thebrush seal of this invention; and

FIG. 9 is a cross sectional view taken generally along line 9—9 of FIG.8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring initially to FIGS. 3–5, the hybrid bush seal 10 of thisinvention is intended to create a seal between two relatively rotatingcomponents, namely, a fixed stator 12 and a rotating rotor 14. In thisembodiment, the seal 10 comprises a first group or bundle 16 of sealbristles 18 and a second bundle 20 of seal bristles 18 which are axiallyspaced from the first bundle 16. As used herein, the term “axial” or“axially spaced” refers to a direction along the longitudinal axis ofthe stator 12 and rotor 14, e.g. axis 22 in FIG. 3, whereas “radial”refers to a direction perpendicular to the longitudinal axis 22.

The seal bristles 18 in each bundle 16 and 20 have an inner end 24 andan outer end 26. In the embodiment illustrated in FIGS. 3 and 4, theouter end 26 of the seal bristles 18 in each bundle 16, 20 is affixed tothe stator 12. For purposes of the present discussion, the constructionand operation of the seal 10 herein is described with the seal bristles18 in that orientation. It should be understood, however, that the innerend 24 of the seal bristles 18 could be affixed to the rotor 14.Preferably, the seal bristles 18 are mounted to the stator 12 or rotor14 by clamping, welding, brazing or other means of affixation. The sealbristles 18 in each bundle 16 and 20 are arranged in an annular shapecorresponding to the circumferential gap between the stator 12 and rotor14. As best seen in FIGS. 4 and 5, a spacer plate 28 is located in theaxial space between the seal bristle bundles 16 and 20. The sealbristles 18 in bundle 16 are captured between a high pressure backingplate 30 associated with the stator 12 and the spacer plate 28, whereasthe seal bristles 18 in bundle 20 extend between a second spacer plate31 and a low pressure backing plate 32.

In one presently preferred embodiment, the seal bristles 18 are formedof a wire material, but it is contemplated that different materials maybe utilized depending upon environmental conditions of the particularsealing application. In the past, brush seal materials, including theseal bristles, were chosen primarily for their high temperature and wearcapability properties. The bristle seals 18 of this invention do notcontact the rotor 14, as discussed below, and therefore different wearcharacteristics and other considerations are involved in the selectionof appropriate materials for the bristle seals 18 as compared toconventional brush seals. The bristle seal 18 geometry may be angled inthe direction of rotation of the rotor 14, or, alternatively, thebristle seals 18 may be straight and have varied angles. The bristleseals 18 may be round, square, rectangular or other shapes, and, ifround, the diameter of each bristle seal 18 can be varied depending onthe nature of the sealing environment. The outer end 26 of the bristleseals 18 in each bundle 16 and 20 may be fused together or free to moveindependently. Further, the number of seal bristles 18 within eachbundle 16 and 20 can be varied with the understanding that more sealbristles 18 generally leads to improved sealing.

The inner end 24 of the seal bristles 18 in each bundle abut one or moreshoes 34 located in sealing relationship to the rotor 14. In theembodiment of FIG. 4, the shoes 34 are formed with axially spaced ridges36 and 38. One side of the bundle 16 of seal bristles 18 abuts the ridge36, and one side of the bundle 20 of seal bristles 18 abuts the ridge38. FIG. 5 depicts a slightly different construction of shoes 34 inwhich the ridge 36 is the same as that in FIG. 4, but a ridge 40 isformed on the shoes 34 in position to contact the opposite side of thebundle 20 of seal bristles 18 compared to the FIG. 4 embodiment. In bothcases, each shoe 34 is attached at discrete locations to the abuttingseal bristles 18 such as by welding, brazing, clamping or other means.The arc length, width, height, geometry and surface characteristics ofthe shoes 34 can be varied to enhance hydrodynamic pressure between therotor 14 and stator 12, to balance the static pressures within thesystem to vary the pressure sealing capabilities of the seal 10 and forother purposes. Preferably, the shoes 34 are made from sheet metalstampings or similar materials, to reduce manufacturing costs.

Referring now to FIGS. 6–9, alternative embodiments of a brush seal ofthis invention are shown. In FIG. 6, a brush seal 40 is shown in which asingle bundle 42 of seal bristles 18 is located between a high pressurebacking plate 44 and a low pressure backing plate 46. For purposes ofthe present discussion, and consistent with the description of theprevious embodiments, an outer end 48 of each seal bristle 18 in bundle42 is mounted to the stator 12 while the inner end 50 extends toward therotor 14. It should be understood that the seal bristles 18 in bundle 42could be affixed to the rotor 14 instead of the stator 12.

In the embodiments of FIGS. 3–5, axial rigidity and radial compliance ofthe seal 10 is provided by the seal bristles 18 in the bundles 16 and 20through their connection between the stator 12 and shoes 34. In theembodiment of FIG. 6, the seal bristles 18 in the bundle 42 need not beconnected to a shoe 34. Instead, a spring element 52 is connectedbetween the high pressure backing plate 44 and the shoe 34. The springelement 52 provides essentially the same resistance to roll over of theseal 40 as the bundles 16 and 20 of seal bristles 18 in the seal 10 ofFIGS. 3–5. Preferably, the spring element 52 is formed of spring steelor other material which is flexible in the radial direction but stiff inthe axial direction.

The embodiment of FIG. 7 depicts a seal 55 which is similar to the seal40 of FIG. 6, except that two axially spaced bundles 56 and 58 of sealbristles 18 are employed instead of one. The bundle 56 of seal bristles18 is retained between a low pressure backing plate 60 and a spacerplate 62, whereas the bundle 58 is retained between a second spacerplate 64 and a high pressure backing plate 66. As in the embodiment ofFIG. 6, the bristles 18 of each bundle 56, 58 need not be connected to ashoe 34. Axial rigidity and radial compliance are provided primarily bya spring element 68 connected between the low pressure backing plate 60and shoe 34, and a second spring element 70 connected between the highpressure backing plate 66 and the shoe 34.

Referring now to FIGS. 8 and 9, a still further embodiment of a seal 72according to this invention is shown. The seal 72 is similar to that ofseals 40 and 55 except for the spring elements 74. Each spring element74 is essentially a rectangular-shaped beam with an outer band 76radially spaced from an inner band 78. One end of each of the bands 76and 78 is connected to a seat 80 formed in the stator 12, and theopposite end of bands 76, 78 mounts to a ridge 82 formed in a shoe 34.The spring element 74 functions to maintain the shoe 34 in sealingrelationship with the rotor 14 in the same manner as the spring elements52 and 68, 70. A bundle 72 of seal bristles 18 is fixed at its outer endto the stator 12, and the inner end of each seal bristle 18 extendstoward the shoe 34 where it may or may not be affixed thereto.

In each of the embodiments of FIGS. 6–9, the seal bristles 18 formessentially a secondary seal. The shoes 34 are maintained in positionwith respect to the stator 12 and rotor 14 by the spring elements 52, 68and 70, and 74, which cooperate with the bristle bundles to resist rollover.

While the invention has been described with reference to a preferredembodiment, it should be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof.

For example, it has been found advantageous to provide a flow path inthe shoes 34 of this invention to assist in balancing static pressure inthe system. This flow path can take the form of a step 84 formed in theshoe 34, as depicted in FIG. 6.

Therefore, it is intended that the invention not be limited to theparticular embodiments disclosed as the best mode contemplated forcarrying out the invention, but that the invention will include allembodiments falling within the scope of the appended claims.

1. A brush seal for sealing a circumferential gap between a firstmachine component and a second machine component which is rotatablerelative to the first machine component about a longitudinal axis,comprising: a bundle of seal bristles each having a first end and asecond end, said first ends of said bundle of seal bristles beingmounted to one of said first and second machine components and saidsecond ends thereof extending in a direction toward the other of saidfirst and second machine components; at least one spring element havinga first end connected to one of said first and second machinecomponents, and a second end extending in a direction toward the otherof said first and second machine components; and at least one shoeextending about one of said first or second machine components inposition to create a non-contact seal therewith, said at least one shoebeing fixed to whichever one of said first and second ends of saidspring element is located opposite said first ends of said sealbristles, said at least one shoe preventing said second ends of saidseal bristles from directly contacting either said first or secondmachine component.
 2. The brush seal of claim 1 in which said at leastone spring element is resilient in the radial direction and stiff in theaxial direction.
 3. The brush seal of claim 1 in which said at least onespring element includes a first spring element and a second springelement which is axially spaced from said first spring element.
 4. Thebrush seal of claim 1 in which said at least one spring element isformed of a strip of spring steel.
 5. The brush seal of claim 1 in whichsaid at least one spring element is a beam having radially spaced innerand outer bands, one end of each of said inner and outer bands beingconnected to one of the first and second machine components and theother end of said inner and outer bands being connected to said at leastone shoe.
 6. The brush seal of claim 1 in which said at least one shoeis formed with a flow path for balancing static pressure.
 7. A brushseal for sealing a circumferential gap between a first machine componentand a second machine component which is rotatable relative to the firstmachine component about a longitudinal axis, comprising: a first bundleof seal bristles each having a first end and a second end, said firstends of said first bundle of seal bristles being mounted to one of saidfirst and second machine components and said second ends thereofextending in a direction toward the other of said first and secondmachine components; a second bundle of seal bristles each having a firstend and a second end, said first ends of said second bundle of sealbristles being mounted to one of said first and second machinecomponents and said second ends thereof extending in a direction towardthe other of said first and second machine components, said secondbundle of seal bristles being axially spaced from said first bundle ofseal bristles; at least one spring element having a first end connectedto one of the first and second machine components and a second endextending in a direction toward the other of said first and secondmachine components; and at least one shoe extending about whichever oneof the first or second machine component is located opposite said firstends of said seal bristles in position to create a non-contact sealtherewith, said at least one shoe being fixed to said second end of saidspring element, said at least one shoe preventing said second ends ofsaid seal bristles of each of said first and second bundles fromdirectly contacting either the first or second machine component.
 8. Thebrush seal of claim 7 further including a high pressure backing plate, alow pressure backing plate and at least one spacer plate located betweensaid high and low pressure backing plates, said first bundle of sealbristles being located between said high pressure backing plate and saidat least one spacer plate and said second group of seal bristles beinglocated between said at least one spacer plate and said low pressurebacking plate.
 9. The brush seal of claim 7 in which said at least onespring element is resilient in the radial direction and stiff in theaxial direction.
 10. The brush seal of claim 7 in which said at leastone spring element includes a first spring element and a second springelement which is axially spaced from said first spring element.
 11. Thebrush seal of claim 7 in which said at least one spring element isformed of a strip of spring steel.
 12. The brush seal of claim 7 inwhich said at least one spring element is a beam having radially spacedbands, one end of each of said inner and outer bands being connected toone of the first and second machine components and the other end of saidinner and outer bands being connected to said at least one shoe.
 13. Thebrush seal of claim 7 in which said at least one shoe is formed with aflow path for balancing static pressure.